Optical fiber communications has generally replaced electrical links over long distances in the past few decades. In more recent past, optical links are being used at shorter distances to connect servers to switches and for datacenters.
The advantages of fiber optics compared to electrical links are the greater bandwidth and reduced degradation of the signal with distance. At 10 Gb/s data rates, for the signal to travel more than 100-300 m in a fiber, generally single mode fiber is needed, with a typical mode size of about 8 microns. As an alternative, when distances are on the order of 100 m or less, multimode fiber and multimode vertical cavity lasers may be used. In this case the core size in the fiber is much larger at about 50 um, and alignment tolerances can be substantially looser. However, the reach is limited as different modes of the fiber travel at different speeds and it is becomes more difficult to transmit multiple wavelength simultaneously.
As bandwidth requirements increase, there is increased parallelism in both single mode and multimode fiber links. In multimode systems, additional fibers may be added to form a fiber ribbon. One great advantage of single mode fiber is that multiple wavelengths can be coupled simultaneously to get a parallel link through a single fiber. Thus a 100 Gb/s signal can be sent through a single mode fiber for many kilometers by using ten channels of 10 Gb/s each, with every lane at a different wavelength. For multimode applications, 120 Gb/s may be transmitted over 100 m using a 12 element array of vertical cavity lasers coupled to ribbon fiber with 12 fibers for transmit and 12 for receive. The parallel ribbon fibers are of course quite expensive and connectors with 24 fibers inside are complicated to make, even if they use multimode fiber with looser alignment tolerance.
Generally it is desirable to minimize the electrical power consumed and hence the heat generated by any optical link, because of the desire to pack the electronics and optics into as compact a space as possible. Therefore it is generally preferred to minimize the electrical drive current for each laser. Vertical cavity surface-emitting lasers (VCSELs), with their very small active area, have a threshold current for lasing typically below 5 milliamps, and can operate at a bias current below 20 milliamps, with a peak-peak modulation current of 10 milliamps or less. These lasers, however, have serious drawbacks in multiple wavelength links. Their lasing wavelengths generally scale linearly with the thickness of the semiconductor layers that make up the laser cavity, and generally cannot be controlled to within the accuracy required in multiple wavelength systems, which is typically 0.1% or better. Also their small area and circular shape give them a high thermal resistance, and their output optical power is generally limited to below 1 milliwatt, especially at high temperature. The optical elements that combine optical signals at different wavelengths into a single fiber inevitably introduce optical loss. In order to ensure that sufficient optical power is transmitted for reliable data transfer, there is generally a minimum output power requirement for the laser, typically in excess of 1 milliwatt.
Greater optical power output and wavelength control can often be provided by edge-emitting mode-controlled semiconductor lasers such as distributed feedback (DFB) lasers. In these lasers, the wavelength is controlled by the periodicity of an etched grating, which is controlled very precisely by lithography, either optical or electron beam. The narrow stripe geometry of DFB lasers is very suitable for heat dissipation, so these lasers can be driven at high current in order to achieve high optical power output. The disadvantage of conventional DFB lasers is they generally use a high electrical current drive relative to VCSELs. The threshold current for lasing of DFBs is typically around 10 milliamps at 25° C. and 25 to 40 milliamps at 85° C.
Unfortunately it is difficult to achieve very low threshold current for a DFB laser, for example by reducing the length of the laser, because the required optical gain per unit length increases beyond what is easily achievable. Another problem with shorter DFB lasers of for example 110 μm length is that it is difficult to cleave devices shorter than about 200 μm.
Regarding possible use of Distributed Bragg Reflector (DBR) laser designs, the disadvantages of this structure relate to the longitudinal optical mode control, since there is no grating in the active region. This type of laser has a certain yield for single-wavelength operation, which means that screening is required. A bigger problem is the tendency to “hop” from one wavelength to another as the drive current is changed. This tendency means that there would be a narrow range of acceptable bias current for any individual laser, so device yield would be quite low and testing and calibration would be time consuming.
The electrical drive circuitry that supplies current to the laser diode is another cause of undesired power dissipation, because of its complexity and the general requirement to provide separate current paths for the direct current bias and the radio-frequency (RF) data signal. The simplest method to modulate a DML for the transmission of binary data is to turn the laser on for transmission of a 1 bit and to turn the laser off for the transmission of a 0 bit. This method only works well for relatively low bit rates (up to Mb/s) as the turn on delay of the laser and associated noise and laser response (laser relaxation oscillation) result in significant degradation of the transmitted signal for bit rates exceeding several Mb/s. Therefore at higher bit rates it is necessary to keep the laser above its lasing threshold condition at all times, and hence the laser is modulated from a low lasing power P0, achieved at laser current I0, for a 0 bit, to a high lasing power P1, achieved at a laser current I1, for a 1 bit, as illustrated in FIG. 1. Laser drivers for DMLs at bit rates exceeding several Mb/s typically consist of a current steering output stage and therefore can drive either zero current for a 0 bit or a nonzero modulation current for a 1 bit. As a result the nonzero current I0 for a 0 bit has to be provided through a different path to the laser. This DC path is typically implemented using inductors to provide high impedance as to block the RF modulation from leaking out through this path, yet be low impedance at DC due to the limited DC supply voltage available. For optical links the RF content of the data covers a wide frequency spectrum resulting in an inductive path that typically consists of several low cost inductors and resistors, or costly high quality inductors. In addition the voltage level for the laser typically does not match the voltage levels for the output stage of the laser driver and as a result the laser driver and laser are AC coupled using a capacitor, and additional inductors are placed on the laser driver side of the AC coupling capacitor to provide the correct DC voltage level on the driver output. For 10 Gb/s this can lead to 10 to 25 passive components in the circuit connecting the laser driver output to the laser as shown in FIG. 2 which shows a directly modulated laser diode 21 driven with a data signal provided by a driver 23, with passive components 25, for example as mentioned above. Some of those components can be of significant size to provide high impedance at relatively low frequencies. This typical bias circuit, usually referred to as a bias tee, is not only large in size but also inefficient in terms of electrical power dissipation, as the DC component of the LDD output stage is not used in the LD due to the DC blocking of the coupling capacitors. In some implementations the differential configuration shown in FIG. 2 is simplified to a single ended configuration, resulting in a lower number of passive components. For applications with multiple 10 Gb/s lanes in the same module, however, a differential configuration would be preferred to reduce link performance degradation caused by electrical crosstalk between the lanes. Variations on this, which add a laser driver final stage co-located in the laser package, use relatively large components inside the laser package, which is undesirable as well.